This invention relates to a power circuit for lamps and, more particularly, to power control circuit for incandescent illumination lamps fitted to an operating panel of an audio system mounted in a car (hereinafter called a car stereo).
Various operating push buttons are provided on a operating panel for the car stereo. These push buttons are lit from their backside respectively with a lamp, so that a driver can easily operate the car stereo. By such illumination the stamped-out characters on the buttons, the indicators, and the circumference of buttons are lit and the function and position of each button can be distinguished. Incandescent lamps have been used for this illumination.
There are two lighting systems of incandescent lamps, that is, a AC lighting system using AC power and a DC lighting system using DC power.
As a conventional power circuit for incandescent lamps used for a car-stereo set, there has been used a DC lighting system which supplies DC voltage to incandescent lamps by direct connection to a battery mounted on a car.
However, the power circuit of the DC lighting system for incandescent lamps is complex as compared with that of the AC lighting system and also has such a problem that life of incandescent lamps is short.
For example, a conventional DC power circuit for incandescent lamps causes a symptom called notching in which with the passage of time the diameter of lamp filaments become irregular. That makes, with the passage of time, irregular the diameter of lamp filaments which are uniform before use, and thus the filament surfaces becomes uneven. Further progress of this symptom burns a thinned part to break it. Hence the notching greatly affects the life of lamps. From the past study it has been proved that the notching is caused by partial crystal growth in the filaments as a result of movement of metal ions in the filament metal along an electric field which is generated in the filaments because the incandescent lamp is lit with the direct current. Such behavior of metal ions is called an electromigration effect.
The purpose of this invention is to provide a lamp power control circuit capable of extending the life of incandescent lamps by reducing the notching of lamp filaments even when a DC lighting system is used for the lamp power circuits.